Is this the resonant EOM or a BBEOM driven with an external circuit? The driving requirements are different, if you overdrive you'll get a bunch of higher harmonics.

Maybe check the modulation depth, make sure it isn't too excessive.  Beta=0.3 should be enough. Error signal Vpp =  (where P0 is the incident power, and G is the PD + mixer gain. Just make sure beta it isn't excessive in producing significant higher order components.  

You want to figure out if the 29.5 MHz harmonic feature is an electrical cross coupling or optical. Check your cable routing, are you getting any cross coupling pickup from collocated coaxial lines?  Pretty sure we got the mode cleaner PDH demodulation stuff right, but check mixer is right power level with 4th order LP filter + 50 ohm termination on input side of the LP filter with a RF compatible T-junction.   

Also, looking back at the design documents for the PMC, it looks like the nominal design frequency considered was ~30 MHz modulation.  Have a look Evan's earlier​ calculations, especially with respect to HOM suppression.  It looks like for 30 MHz the 60 MHz harmonic is in a relatively low transmission point for HOM. If the issues are optical artifacts transmitted it might be HOM... you can always up the frequency to something closer to 30 MHz.

Take all of the above with a grain of salt.  Consider how much time to sink into it based on whether you think its actually driving a noise source in the system.  Eliminating this 14.75 MHz harmonic may be in the basket of diminishing returns for your time.  The 36 MHz and 37 Mhz AM components are a much bigger concern for FSS related coupling of RIN into frequency noise.